If he had told her earlier, she wouldn't be this angry now.
But he was very angry at the time, angry that she got engaged to him, married him, and still went to seduce 'him'. He hadn't considered that he might have feelings for her in the future.
If he had known he would develop feelings for her, he swore he would never have kept the truth hidden.
...
Shangguan Anna felt very strange; ever since she thought she saw Shen Haoze lurking near the potted plants that day, she felt like she was being watched no matter what she did in the following days.
The gaze felt particularly intense, making her feel slightly creeped out.
That's when she thought of telling her second brother.
However, every time she went to look for Shangguan Xiu or An Xiaomian, she found that her brother was wrapped up with An Xiaomian in their room, sweet and loving and paying no attention to anyone else.
